Well to analyse what has happened, what have you changed within these past days? not much really yeah?

This is defiantly a software problem just make sure you don't have anyone running on the network whilst you do this; try and reset all permissions on your router / modem and see what tries to connect, if it is any other mac address, check that no other services (task manager) are running that you don't know of.

Also when this is done, make sure no permissions are set like to block certain ports / open some ports etc.

I don't know if that will help to much but it might give you and indication of whats connected.

I think that's really all you can do and make sure you have MSE installed, try and open the ports that Steam says to open when this happens (I believe an option will come up to resolve the issue?

I had this problem and my router created a rule because Steam was opening ports that were to high for normal (torrent ports) and it automatically closed them for safety.

So the changes I made, as far as I can remember, were I:
installed MSI Afterburner/Kombustor
changed my SSD from IDE to AHCI (well, I tried....but it never worked)
I installed League of Legends
I installed some Water Cooling stuff
I've done an "Upgrade" W7 Install, a "Custom" W7 install, and a full wipe install of W7. All W7 installs have been Enterprise

So the changes I made, as far as I can remember, were I:
installed MSI Afterburner/Kombustor
changed my SSD from IDE to AHCI (well, I tried....but it never worked)
I installed League of Legends
I installed some Water Cooling stuff
I've done an "Upgrade" W7 Install, a "Custom" W7 install, and a full wipe install of W7. All W7 installs have been Enterprise
That's everything I can think of.

Did you try the other laptop, like one that hasn't been on your network and try steam from that device.

If not I think someone with a bit more of a networking background should help you.

Also, there's NO router. There's NO modem. I'm hard wired into the wall. I've also confirmed that it works on another computer and NOT on my desktop. I've also tried creating a new profile on my desktop through Steam and got the same error. I know it's my computer, for a fact.

Also, there's NO router. There's NO modem. I'm hard wired into the wall. I've also confirmed that it works on another computer and NOT on my desktop. I've also tried creating a new profile on my desktop through Steam and got the same error. I know it's my computer, for a fact.

ohk thanks, I don't think any drivers would have changed that then, what did you install before this happend? like a week or so before?